Security at Trailhead

You should be very worried, especially on the Tongariro Crossing. The situation there is shocking. There is almost more windscreen glass in the carparks than gravel. Campervans are especially targets because there are people who prey on, supposedly, wealthy tourists. I suggest you leave your vehicle at Turangi, there are operators there who will care for your vehicle and shuttle you to the trailhead. Alternatively it would be safer to leave your vehicle at the Chateau Tongariro and start and finish your trip from there. The carpark there is in a busy centre of population and is less likely to be interfered with.

Yes, most road-ends in New Zealand tend to be generally safe from vandalism, but unfortunately the few that get hit a lot tend to be ones that get large numbers of tourists. As much as authorities are trying to change things, the Tongariro Alpine Crossing still has a reputation as one of the least secure places in NZ to leave a vehicle. If you're looking to do the Tongariro Crossing, there's at least one business in the area that will store your car securely and drive it to either end to drop you off and collect you. When I was there we stayed at Tongariro Holiday Park, who have a car ferrying service. They're 15 minutes from one end and 5 minutes from the other. I think they charged about $40 or $50, which we split between 4 people, and I think it varies depending on whether you want them to take you there in their own vehicle or in your vehicle (as petrol is taxed a lot in NZ and therefore more expensive than some places). See http://www.tongarirocrossing.co.nz/Tongariro_Holiday_Park/Car_Ferrying_IDL=1_IDT=594_ID=3328_.html According to their website you don't have to be staying there to use the service. I couldn't say for sure if they'll do camper-vans, but you could certainly ask, or just let them use their own vehicle and pay more (if they do that which I think they do). Another option would be to stay in a place like Turangi, and use one of the bus services to both ends of the Tongariro Crossing. On the side, I'd suggest having a reserve day for the Tongariro Crossing so you don't feel compelled to do it on the only day you have, and have something else in mind to do on whichever day you don't use. Sometimes the weather gets really bad and potentially dangerous, but lots of people go anyway (often without appropriate gear) because they've only factored in a single day, and they've been looking forward to it for ages. If you have some flexibility in your plan, it could help. If you're going anywhere near Collingwood when you do the Heaphy Track, Somerset Backpackers is awesome. See http://www.backpackerscollingwood.co.nz/ being dropped off and collected.

Sadly there's little doubt that vehicle break ins are problem at some trail ends. Advice to arrange transport to road ends does not solve the problem and adds extra expense, though unavoidable on the non linear walks. Tongariro Northern circuit though is a loop and the best place to leave a vehicle would be near the Whakapapa visitors centre. Best advice is not leave any valuables in the vehicle and certainly not visible. I'm just back from doing some hiking on the South Island, we hired a car as I was worried about leaving a campervan overnight and thought that a car would be less trouble at least the boot was lockable. Also when I planned our itinerary after the nights in hut, we going to save a great deal over backpacker/hostels anyway, an the latter are more convenient, though less flexible I guess.